team: tame false positives

[pierrick-fonquerne]

Scope

• Confirm a finding only when at least two verification lenses respond and none contest it (strict quorum). A single contestation, or fewer than two responses, leaves the finding contested.
• Give each lens the real file content around the finding (a windowed head-file view), not just the patch, so a genuine issue whose evidence sits outside the diff is no longer dismissed. The head sha is fetched once per run and reused for inline posting.
• Scope the security agent to added lines and document common false positives (CI secret references treated as expected usage, untouched permissions out of scope, theoretical issues excluded).

Acceptance criteria

• A finding with one contesting lens is shown as contested, not confirmed.
• A finding verified by fewer than two lenses is contested with an explicit reason.
• Lenses receive the head-file window when the file is line-located and readable.
• fmt + clippy (all/pedantic, -D warnings) + tests are green (32 tests).

Closes #9
Closes #10